The present, invention relates to a misfire diagnosis apparatus for an internal combustion engine, particularly to a stopping system of the misfire diagnosis apparatus when a motor vehicle is in an accelerating condition.
In general, stable power output is desirable at any stroke for a multi-cylinder engine. However, combustion in each cylinder is liable to vary by the following reasons:
(1) Intake air to each cylinder is not uniformly distributed because of complicated configurations of intake pipes and interference of induced air.
(2) A difference of temperature between cylinders caused by a different route of a cooling system.
(3) A slight volumetric difference due to manufacturing allowance between each cylinder and piston.
(4) Slight discrepancy of air fuel ratio for each cylinder caused by unequal fuel injection quantity according to the manufacturing errors of injectors, etc.
Heretofore, the so-called combustion fluctuation has been suppressed to the minimum by the air fuel ratio controls and ignition timing controls of the individual cylinders. In a recent high-performance engine tending toward a higher output and a lower fuel cost, however, when any of the injectors, ignition plugs etc. has degraded or broken down, intermittent misfiring arises and incurs lowering in the output.
Even when the intermittent misfiring has occurred in one cylinder in the multicylinder engine, a driver often drives an automobile without noticing the misfiring. Besides, it is difficult to diagnose during driving the cause of the misfiring is temporary or is the degradation or the like of any of the injectors, the ignition plugs etc.
Therefore, according to the official gazette of Japanese Patent Application Laid-open No. 258955/1986 by way of example, a comparison is made between the difference of the minimum value and maximum value of the r. p. m. of an engine corresponding to a cylinder at the previous combustion stroke and the difference of the minimum value and maximum value of the engine r. p. m. corresponding to the cylinder at the present combustion stroke. The combustion condition of the pertinent cylinder is discriminated, depending upon whether or not the discrepancy between the compared values falls within a preset reference value. In a case where abnormal combustion has occurred in excess of a predetermined number of times, a misfiring is judged, and warning is given.
Meanwhile, when a throttle valve is abruptly opened at, e.g., a starting acceleration, the quantity of intake air to be fed into each cylinder increases, and a temporary misfiring is liable to occur.
With the prior-art technique mentioned above, the misfiring is diagnosed even in the regions of the starting acceleration, etc., so that the diagnosis is liable to become erroneous and poses a problem in reliability.